A tufting machine, especially a tufting machine adapted for the manufacture of carpet, has a pair of pin rollers which are driven to feed a primary backing material off of a large storage roll and over a bed frame. The two pin rollers are disposed on opposite sides of the bed frame so that the first pin roller introduces the primary backing material into the tufting machine, and the second pin roller removes the backing material from the tufting machine. A set of needles are located above the bed frame across the width of the tufting machine and are threaded with yarns. The needles are reciprocated through the action of a needle bar so as to insert the yarns through the primary backing material to form tufts on the face of the primary backing. The tufting machine may have various combinations of loopers and knives to enable the manufacture of loop pile or cut pile bights of yarn on the face of the carpet. Based on the arrangement of threaded needles, loopers and knives, and based on the color of the yarns threaded in the needles, the tufting machine can generate various patterns of yarn bights.
In a conventional mechanical tufting machine, the second pin roller, or exit pin roller, is driven off of a main drive shaft by a pulley and belt arrangement, and the first pin roller, or entry pin roller, is driven off of the exit pin roller by another pulley and belt arrangement. The exit pin roller is driven at a slightly faster speed so as to produce tension across the primary backing material and to insure that the primary backing material is continuously advanced over the bed frame. In addition to the pin rollers, the other parts of a conventional mechanical tufting machine, such as the needle bar and loopers, are also driven off of the main drive shaft.
In these conventional tufting machines, it is necessary to synchronize the feed of the backing material across the bed frame with the speed of reciprocating needles to produce a pre-determined number of stitches per inch in a longitudinal direction of the backing material. In such tufting machines, it has been necessary to change the sheaves of the gear box connected to the entry and exit pin rollers on the tufting machine in order to change the number of stitches per inch. As a result, it was traditionally difficult to change the number of stitches per inch being sewn by the tufting machine, for instance, to arrive at a pre-determined weight for a square yard of carpet. Furthermore, it was practically impossible to provide for different length stitches within the same pattern without utilizing crammed sheaves or other notoriously complicated mechanical arrangements such as described in Ingram, et al., U.S. Pat. No. 4,577,208. These arrangements provided no means for fine tuning the lengths of the varied stitches in the pattern as is typically required if two rows of needles are utilized in the pattern. Also the sheer complexity of the arrangements generally has required operation of conventional tufting machines at slower speeds, and has provided only limited pattern variations.
One development that has enabled greater variability for the backing feed drive is that of a computer controlled tufting machine as exemplified by Taylor, U.S. Pat. No. 5,005,498. Modern computer controlled tufting machines use separate servo motors to drive the entry and exit backing feed rolls in ratio to the speed of the main drive shaft. While these computer controlled servo motor driven backing feed rolls have provided a straightforward solution to the problem of changing stitch density, and have provided greater versatility in controlling the backing feed, they did not suggest utilizing a variable stitch rate in tufting fabrics. Similarly, the invention of Ingram, U.S. Pat. No. 4,577,208, while providing a variable stitch rate, also correspondingly produced varied stitch density. While such patterning is useful in some instances, most carpet is preferred with a relatively uniform stitch density.